The ability to detect the presence of air flow is a key element of many systems and devices. For example, most combustion systems require a means for sensing air flow to ensure that sufficient air is being provided for proper combustion. Likewise, many industrial and manufacturing processes depend on sensors to monitor air movement, such as baking and curing processes that utilize air flow to evenly distribute heated air. Still another use for air flow sensors is monitoring exhausts of various equipment to ensure that the exhaust system is functioning properly.
Air flow sensing is also an essential element of numerous medical applications. For example, air flow sensors may be used in conjunction with control electronics to form a breath-actuated electrical interface which enables paraplegics to control a wide variety of electrical, electronic and electromechanical devices.
Air flow sensors are particularly desirable for use as inhalation sensors in association with pulmonary medication delivery devices. Delivery of medication via inhalation offers several advantages over other methods of medication delivery. For example, inhalation is less invasive to the patient than intravenous or intramuscular injection, which requires piercing of the patient's skin. Such injections cause discomfort for the patient and may also increase the risk of infection. Delivery of medications orally also suffers from various drawbacks, such as low absorption rate, relatively low absorption level, and potential incompatibility with many patients, particularly those with digestive disorders. Similarly, transdermal patches have a relatively low absorption rate and low absorption level. In contrast, inhalation delivery is non-invasive, reducing patient discomfort and the risk of infection while providing a high absorption rate and a high absorption level. In addition, medications may be delivered via inhalation in many cases when the patient is unable to orally ingest medications.
Several obstacles must be overcome to effectively deliver medications via inhalation. Firstly, the medication must typically be stored in a solid, liquid or powder form and then aerosolized. The aerosolized medicine must then be mixed with breathing air at an appropriate concentration and air pressure to facilitate efficient delivery, yet not interfere with the patient's breathing. In addition, the mixing of the medication and the air must be controlled such that the patient is provided a known dosage. Finally, the air-medication mixture must be delivered to the patient with a minimum of loss, such as by leakage, since lost medication results in wastage and reduced accuracy in dosage measurement.
Air flow sensors, also known as “breathing sensors” and “inhalation sensors,” are frequently used in conjunction with inhalation devices to synchronize the release of medication with inhalation. Synchronized release is desirable to ensure delivery of the medicine and minimize waste, since the medication is delivered only during inspiration.
A number of devices have been developed to measure air flow, with some success. A well-known air flow sensing device is a vane-actuated or “sail” switch, such as used with the inhaler disclosed by Mecikalski et al. in U.S. Pat. No. 5,577,497. An electrical switch is coupled to an actuator that is adapted to be displaced by air flow. The actuator is typically lightweight and includes a surface area arranged to at least partially block an airway such that the actuator is moved due to pressure exerted against it by flowing air, resulting in actuation of the switch. Although sail switches are in common use, they suffer from a number of shortcomings. For example, sail switches are difficult and cumbersome to set for actuation at a particular desired airflow level. This is due to the criticality of the actuator's position in the airway for proper operation, coupled with the inherent mechanical variations present in airways, actuators and electrical switches. In addition, variations in air flow can cause erratic actuation of the switch. Further, sail switches are susceptible to the vibration and shock typically encountered during normal handling, which can cause unintended actuation or undesired changes to the switch's actuation setpoint.
Another well-known air flow sensor is a pressure transducer, such as used with the medication dispenser disclosed by Johansson et al. in U.S. Pat. No. 5,392,768. The pressure transducer detects air flow by measuring pressure changes with resistive or piezoresistive strain gauges that are implanted on a membrane or diaphragm. The membrane or diaphragm is displaced by air flow, and the displacement is indicated by a change in the electrical value of the strain gauge. Although pressure transducers overcome many of the mechanical limitations of sail switches, they also have a number of limitations. In particular, pressure transducers suffer from thermal and long-term drift, reducing the accuracy of the pressure switch and necessitating the use of various compensation measures, such as expensive narrow-tolerance electronic components and offset compensation circuitry or software programs. In addition, the output signal of pressure transducers may vary with the orientation of the transducer, further reducing the accuracy and/or repeatability of the air flow sensor's actuation setpoint.
Use of a hot-wire anemometer or mass flow sensor to measure air flow is also common in the art, such as used with an inhaler disclosed by Robertson et al. in U.S. Pat. No. 5,487,378. A resistive wire is electrically heated to a predetermined temperature. As air flowing around the heated wire cools it, the electrical current flowing through the wire is increased to return it to the predetermined temperature. Since the amount of air moving around the wire is directly related to the amount of cooling experienced by the heated wire, a feedback arrangement may be established whereby the current flowing through the wire is measured to sense whether or not air is flowing. However, the cooling effect of the air can vary, depending on the velocity, temperature, humidity and density of the air, reducing the accuracy and/or repeatability of the air flow sensor's actuation setpoint under varying environmental conditions. In addition, relatively complex electronic circuitry is required to convert the electrical current flowing through the wire to a logical signal that indicates whether or not air is flowing.
The prior art also includes angular displacement sensors and flexible potentiometers (collectively termed “flexible sensors” herein) that utilize a resistive ink screened or deposited onto a flexible substrate. The resistance of the resistive ink changes when the substrate is flexed, providing an electrical indication of the displacement of the substrate. Examples of flexible sensors are disclosed by Langford in U.S. Pat. No. 5,157,372 and by Gentile et al. in U.S. Pat. No. 5,086,785. However, prior art flexible sensors suffer from a relatively high cost due to the process steps and materials required to place a low-resistance conductor over the resistive ink to lower the nominal resistance of the flexible sensor. The low-resistance conductor also adds to the thickness of the flexible sensor, reducing its flexibility and thus limiting the flexible sensor's ability to detect relatively low levels of air flow.
Other air flow sensing devices are available in the art, such as thermally sensitive resistors, thermally sensitive crystals and piezoelectric actuators. However, these devices likewise suffer from at least some of the mechanical, electrical and environmental limitations of the aforementioned devices.
Several of the aforementioned air flow sensors have been used in conjunction with breath-actuated pulmonary medicine delivery devices, with some success. However, the drawbacks associated with these sensors can result in greater manufacturing expense, reduced accuracy and/or repeatability under varying environmental conditions, and a need for careful handling.
There is a need for an air flow sensor capable of operating accurately and repeatably under varying environmental conditions. There is a further need for an air flow sensor that is robust and capable of withstanding normal handling and orientation without degradation in performance. There is a still further need for an air flow sensor that does not require complex electronic circuitry. There is a particular need for an air flow sensor capable of reliably and repeatably sensing air flow resulting from a patient's inhalation to trigger a handheld drug delivery device to deliver a known dosage of medication.